Saat SI yang dikira piawaian waktu atom telah didefinisikan berdasarkan suatu sejarah going back to the former standard time scale of waktu efimeris (ET). It can now be seen to be close to the average second of 1/86400 of a mean solar day antara 1750 dan 1892. Saat SI telah didefinisikan pada 1967, sebagai 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. Nombor ini first arose from calibration of the caesium standard by the second of ET: in 1958, the second of ET was determined as the duration of 9,192,631,770 ± 20 cycles of the chosen caesium transition,[4] (while at about the same time, dan with the same caesium standard, the then-current mean length of the second of mean waktu suria (UT2) had been measured at 9,192,631,830 cycles).[5] Later verification showed that the SI second referred to atomic time was in agreement, within 1 part in 1010, with the second of ephemeris time as determined from lunar observations.[6] Time as measured by Earth's rotation has accumulated a delay with respect to atomic time standards. Sejak 1961 hingga 1971, the rate of (some) atomic clocks was (for purposes of UTC) constantly slowed to stay in sync with Earth's rotation. (Before 1961, broadcast time was synchronized to astronomically determined Greenwich Mean Time.) Sejak 1972, saat pancaran radio adalah sama dengan piawai saat SI yang dipilih pada 1967.

UTC is counted by atomic clocks, but is kept approximately in sync with UT1 (mean waktu suria) dengan memasukkan saat lompat jika perlu. Ini berlaku apabila perbezaan (UT1 − UTC) menghampiri 0.9 saat, dan biasanya dimasukkan pada akhir Jun 30 atau Disember 31 (walaubagaimanapun saat lompat boleh dimasukkan pada akhir apa-apa bulan). Pada 1 Januari, 1972, the initial offset of UTC from TAI was chosen to be 10 saat, which approximated the total difference which had accumulated since 1958, when TAI was defined equal to UT2, a smoothed version of UT1 (GMT) no longer used. The table shows the number of leap saat added since then. Jumlah beza antara TAI dan UTC adalah 10 saat more than the total number of leap saat.

Tidal braking memerlahan pusingan Bumi, menyebabkan bilangan saat SI dalam mean solar day to increase by approximately 2 millisaat sekurun (meaning a projected increase from the current 86400.002 to 86400.004 by the early part of the 22nd century). Additionally, events or processes that cause a significant change to the mass distribution of the earth, thereby changing its moment of inertia, can affect the rate of rotation due to conservation of angular momentum. Most notable in recent times is the Gempa bumi Laut India 2004 which, according to theoretical models, is thought to have decreased the solar day by 2.68 microseconds[7]. For unknown reasons, the slowing of the earth's rotation decreased in 1999, so the mean solar day has become 1 ms shorter dan fewer leap saat have been needed after year 2000.[8] One should note that this does not mean that the earth sped up (other than the small, short-lived "hills" visible on the graph around 2004 dan 2005), it simply means that the rate of slowing decreased, so that the difference between UTC dan UT1 approaches .9 saat less often.

The International Earth Rotation dan Reference Systems Service (IERS) announces the insertion of a leap second whenever the difference between UTC dan UT1 approaches 0.6 s, to keep the difference between UTC dan UT1 from exceeding 0.9 s. IERS publishes announcements every six months, whether leap saat are to occur or not, in its "Bulletin C". Such announcements are typically published well in advance of each possible leap second date — usually in early Januari for June 30 dan in early July for Disember 31. Because the Earth's rotation rate is unpredictable in the long term, it is not possible to predict the need for them more than six months in advance.

The most recent leap second was added at the end of Disember 31, 2008.[9][10]

After 23:59:59 UTC, a positive leap second at 23:59:60 would be counted, before the clock indicates 00:00:00 of the next day. Negative leap saat are also possible, should the Earth's rotation become slightly faster — in which case, 23:59:58 would be followed directly by 00:00:00 — but they have not yet been used. Leap saat occur only at the end of a UTC month, dan have only ever been inserted at the end of June 30 or Disember 31. Unlike leap days, they occur simultaneously worldwide; for example, the leap second on Disember 31, 2005 occurred at 23:59:60 UTC. This was 18:59:60 (6:59:60 p.m.) U.S. Eastern Standard Time dan 08:59:60 (8:59:60 a.m.) on Januari 1, 2006 Japan Standard Time.

Pada 5 Julai, 2005, the Head of the Earth Orientation Center of the IERS sent a notice to IERS Bulletins C dan D subscribers, soliciting comments on a U.S. proposal before the ITU-R Study Group 7's WP7-A to eliminate leap saat from the UTC broadcast standard before 2008. (The ITU-R is responsible for the definition of UTC.) The Wall Street Journal noted that the proposal was considered by a U.S. official to be a "private matter internal to the ITU", setakat July 2005[update][11]. It was expected to be considered in November 2005, but the discussion has since been postponed.[12] Under the proposal, leap saat would be technically replaced by leap hours as an attempt to satisfy the legal requirements of several ITU-R member nations that civil time be astronomically tied to the Sun.

Several arguments for the abolition have been presented. Some of these have only become relevant with the recent proliferation of computers using UTC as their internal time representation. For example, currently it is not possible to correctly compute the elapsed interval between two instants of UTC without consulting manually updated dan maintained tables of when leap saat have occurred. Moreover, it is not possible even in theory to compute such time intervals for instants more than about six months in the future. This is not a matter of computer programmers being "lazy"; rather, the uncertainty of leap saat introduces to those applications needing accurate notions of elapsed time intervals either fundamentally new (dan often untenable) operational burdens for computer systems (the need to do online lookups) or insurmountable theoretical concerns (the inability in a UTC-based computer to accurately schedule any event more than six months in the future to within a few saat).

Beberapa bangkangan terhadap cadangan itu telah disuarakan. Dr. P. Kenneth Seidelmann, editor of the Explanatory Supplement to the Astronomical Almanac, wrote a letter[13] lamenting the lack of consistent public information about the proposal dan adequate justification. Steve Allen of the University of California, Santa Cruz cited the large impact on astronomers in a Science News article.[14] He has an extensive online site[15] devoted to the issues dan the history of leap saat, including a set of references about the proposal dan arguments against it.[16]

Arguments against the proposal include the unknown expense of such a major change dan the fact that universal time will no longer correspond to mean waktu suria. It is also answered that two timescales that do not follow leap saat are already available, International Atomic Time (TAI) dan Global Positioning System (GPS) time. Computers, for example, could use these dan convert to UTC or local civil time as necessary for output. Inexpensive GPS timing receivers are readily available dan the satellite broadcasts include the necessary information to convert GPS time to UTC. It is also easy to convert GPS time to TAI as TAI is always exactly 19 saat ahead of GPS time. Examples of systems based on GPS time include the CDMA digital cellular systems IS-95 dan CDMA2000.

At the 47th meeting of Civil Global Positioning System Service Interface Committee in Fort Worth, Texas, it was announced that a mailed vote would go out on stopping leap saat. Rancangan undian itu adalah:[17]

↑:(1) In "The Physical Basis of the Leap Second", by D D McCarthy, C Hackman dan R A Nelson, in Astronomical Journal, vol.136 (2008), pages 1906–1908, it is stated (page 1908), that "the SI second is equivalent to an older measure of the second of UT1, which was too small to start with dan further, as the duration of the UT1 second increases, the discrepancy widens."

(2) Pada akhir 1950-an, piawaian caesium digunakan untuk mengukur both the current mean length of the second of mean waktu suria (UT2) (measured over a period of about a year) (hasil: 9192631830 kitaran, probable error not stated), dan juga saat waktu efimeris (ET) (measured over a period of about 3 years) (hasil:9192631770 ± 20 kitaran): lihat "Time Scales", by L. Essen, in Metrologia, vol.4 (1968), pp.161–165, on p.162. As is well known, the 9192631770 figure was chosen for the SI second. L Essen in the same 1968 article (p.162) stated that this "seemed reasonable in view of the variations in UT2".

↑"Extra Second Will be Added to 2005". LiveScience. Diperoleh pada November 26, 2009. "dan then, in 1999 for reasons still unknown, the rotation of the Earth speeded up a bit, so we haven't had to add a second since then," O'Brian told LiveScience in a telephone interview.